1. Field of the Invention
The present invention relates to improvements in a semi-insulating compound semiconductor device.
2. Description of the Related Art
Semi-insulating compound semiconductor devices (hereinafter also briefly referred to as "semiconductor device") fabricated with a semi-insulating compound semiconductor substrate (hereinafter also briefly referred to as "substrate") such as the GaAs semiconductor substrate are widely used as linear ICs or digital ICs. Such semiconductor devices are also widely used, in the form for example of the field-effect transistor (FET), in high-frequency amplifiers, frequency dividers, logic ICs, and so on. Usually, the semiconductor device is formed of an input terminal portion and an element formation region, and the input terminal portion is provided with a protection diode for improvement of the withstanding voltage.
Now, it is reported that, when the bias for the input terminal of a semiconductor device is under a certain condition, the drain current (I.sub.D ) or the drain conductance (g.sub.m) oscillates at a frequency around 1 Hz to 1 kHz. Refer to "Low Frequency Oscillation In GaAs IC's", Daniel Miller et al, GaAs IC Symposium-31, pp. 31 -34, 1985.
Though the cause of the above phenomenon has not been completely clarified as yet, it is generally understood that a leakage current is injected into the substrate through the protection diode provided at the input terminal portion of the semiconductor device and the leakage current modulates the potential at the interface between the channel operation region of the semiconductor device and the substrate. As a result, the low-frequency oscillation of the drain current I.sub.D or drain conductance g.sub.m occurs. Such low-frequency oscillation, in turn, causes low-frequency noises in a linear IC or malfunction of a digital IC.